This invention relates generally to cable television tuning systems. It relates more particularly to up-down frequency converters for use in cable television systems.
Cable television systems typically comprise a central master source of television programming information which imparts a program or group of programs at different assigned frequencies to a network of cable connections which, through various distribution and selection network components, ultimately terminate in a branched out plurality of user terminals typically in residences or commercial establishments. Particularly in great metropolitan areas cable systems may comprise one or more master sources which are patched into a common distribution system. In the typical cable television environment television signals are simultaneously broadcast over different assigned carrier frequencies normally from 50 to 450 megahertz. An increasingly popular aspect of cable television environment is the commercial development known most popularly as subscription television systems. Such systems are functionally similar to publicly available cable television environment with the exception that an added feature within the distribution system provides for scrambling the television signals at some point in the distribution system prior to the user terminal in combination with a mechanism at the user terminal by which a user upon becoming a subscriber may then be provided with an operative signal.
While there are several types of subscription television systems operating both over the air and through cable, they all share the common elements of several known systems. In one such system the signal broadcast or imparted to the cable system is "scrambled" by some preferred method, is received at the subscriber in a scrambled condition, and is processed by an "unscrambler" device which is interposed between the signal input and the subscriber's television receiver. In another known system the signal itself is transmitted in a normal fashion but selectively impaired by individually controllable transmission devices interposed within the distribution system. In this latter system the subscriber is provided with an operative signal in response to a transmission command within the inaccessible distribution system.
The signal processing systems and sub-systems operating within a cable television distribution network function under a number of varying and demanding conditions. For example the great number of television channels which are simultaneously broadcast over the system means that the signal processing devices of the system must be able to process a wide range of frequency signals. In addition, a number of sources of possible interfering signals from non-cable broadcast media often at the same or very proximate signal frequencies as those being carried by the cable system can increase the chances of interference.
As mentioned above, the broad range of signals simultaneously broadcast over cable systems is typically from 50 to 450 megahertz. A television tuner capable of directly processing signals in the range of 50 to 450 megahertz would be difficult if not impractical to provide especially since the problems of cross-modulation and intermodulation would be great. Accordingly, practitioners in the art employ what has become to be known as an up-down tuner system. The operation of an up-down tuner forms a very important portion of the present invention and will be explained in greater detail below. The operation of an up-down converter may be understood to be the utilization of an initial frequency conversion stage which frequency up converts the incoming selected signal to an intermediate frequency signal of a frequency higher than the highest received signal, that is, above 450 megahertz. This high frequency IF signal is then frequency down converted by a second frequency converter stage to a selected commercially utilized television broadcast signal frequency. Most typically the selected broadcast signal frequency corresponds to one of the lower VHF channels not in use in the cable locale.
Such frequency conversion devices generally form a portion of a total television system as for example in cable or subscription television distribution systems. When so used, it is advantageous because of the construction of such signal processing systems permits separate packaging the cable television up-down converter. That is, a single sub-system unit may be provided to which the incoming signals are applied and within which the frequency up-down conversion takes place. There is also an overriding aspect of such system design to provide for isolation between the input and output signals and first and second local oscillators signal frequency to avoid the generation of undesired interferring signals and signal products. Optimum package design in order to assure that the cable television up-down tuner most advantageously adapts to the remainder of the distribution network requires that the overall package size be maintained as small as possible. Further, it is advantageous to have both the input and output cable connections on the same facet of the panel face to provide ease of tuner installation within the remainder of the distribution device.
Unfortunately, however, several of these desirable qualities are somewhat hostile to each other. For example it has been found that maintaining a small package size with input and output connections on the same side necessarily brings input and output terminals closer together and thus increases the possibility of interference. Each reduction of package size necessarily reduces interior spacing of components within the unit and also increases the likelihood of internal feedback problems within the unit. These difficulties are further increased with greater signal amplitude and higher magnitude of gain for the up-down converter itself. In view of the steady requirement trend toward size reduction of cable television distribution components, these remains a need in the art for an up-down converter which may be packaged as a small unit with input and output terminals located on the same cabinet face while still being capable of handling high amplitude signals and having sufficiently high gain to be compatible with the remainder of the system.